An avalanche photodiode (APD) used as a photodiode having multiplying effect is well-known.
The avalanche photodiode comprises a photo-electric conversion region which imparts a photo-electric effect after exposure to an incident light and a multiplying region which imparts an avalanche effect by applying a reverse bias voltage thereto. A desired distribution of impurities in the avalanche photodiode is dependent upon using electronsor positive holes as carriers for the multiplying step.
In order to impart the desired functions of the avalanche photodiode, it is necessary to form the photo-electric conversion region from a substance that produces a photo-electric effect upon exposure to incident light and the energy gap thereof should be smaller than the photon energy (h.nu.).
On the other hand, the multiplying characteristic of the carrier region is dependent upon a ratio of the impact ionization coeffecient of electrons to that of positive holes in the multiplying region.
It is known that the avalanche photodiode uses a substance for providing .alpha./.beta.&gt;&gt;1 or .alpha./.beta.&lt;&lt;1 (.alpha. and .beta. are respectively the impact ionization coefficient of the electrons and that of the positive holes) and uses a carrier for providing a higher absolute value for the impact ionization coefficient.
However, the conventional avalanche photodiode is made of a single crystalline substance and it is essential that the avalanche photodiode have the photo-electric effect as one of its functions. As a result of this, it is necessary to select a substance having an energy gap smaller than the photon energy (h.nu.). Accordingly, it has not been possible to effectively use a substance for providing .alpha./.beta.&gt;&gt;1 or .alpha./.beta.&lt;&lt;1--the impact ionization coefficient ratio in the carrier multiplying region.
That is, it has not been possible to provide an avalanche photodiode having high quantum efficiency and low noise, to provide a satisfactory charging effect after exposure to the incident light; and to provide .alpha./.beta.&gt;&gt;1 or .alpha./.beta.&lt;&lt;1--the impact ionization coefficient ratio.